Field of the Invention
1. The present invention relates generally to dilution of an influent slurry stream entering into a settling tank or basin and, more particularly, to a method and apparatus of effecting self-dilution of such slurry with improved control regarding the flow and flocculation of the slurry stream.
State of the Art
2. Various techniques may be used in the separation of suspended solid particles from the liquid in which they are suspended. These techniques may include, for example, flotation, filtration, centrifugation, expression and sedimentation.
Conventionally, the technique of sedimentation includes introducing a slurry (i.e., a liquid containing suspended solid particles) into a settling tank or basin of, for example, a clarifier or thickener, and allowing the suspended solid particles to settle by gravity to form a sludge or thickened mud on the bottom of the tank and a clarified liquor at the top of the tank. The sludge may then be collected and further processed or otherwise disposed of. Likewise, the supernatant liquid, also referred to as the clarified liquor, may be collected for further processing or disposal, or possibly reused to assist in a similar separation process.
In introducing the influent stream of slurry into the settling tank, it is desirable to control the flow of such a stream so as to avoid, or at least minimize, the disruption of the sludge formed along the bottom of the tank. Undue disruption of the sludge causes particles to become suspended within the supematant liquid, once again resulting in an inefficient sedimentation process. In an effort to control the flow and distribution of the influent slurry, a feedwell may be positioned to receive the slurry as it is introduced into the settling tank. Conventionally, a feedwell includes an area or compartment within the settling tank but which is separated from the contents of the settling tank. The influent stream of slurry is then directed along a predetermined flow path to dissipate the kinetic energy associated with the flow of the influent stream of slurry. This is done so that the influent stream has reduced flow characteristics as it leaves the separated compartment and is intermixed with the contents of the settling tank. Furthermore, the feedwell may be configured to control the distribution of the influent stream into the settling tank such that the influent stream is not introduced at a single location within the settling tank. In essence, the feedwell reduces the velocity of the influent stream and provides increased distribution of the influent stream such that any associated turbulence, which would likely disrupt the sludge formed along the bottom of the settling tank, is reduced or eliminated.
In addition to controlling the flow and distribution of the influent stream of slurry, the feedwell may also be utilized for introducing a flocculating reagent into the influent stream. The addition of a flocculating reagent is sometimes used to expedite the sedimentation process. The flocculating reagent conventionally has a polymeric structure and acts to form a coagulated mass of the suspended particles, the mass sometimes being referred to as a floc. The floc exhibits an increased density over the suspended solid particles and thus provides an improved rate of settling.
While the use of a flocculating reagent may be beneficial in the sedimentation process, various factors may affect its efficiency. For example, it is important to thoroughly mix the flocculating reagent with the influent stream of slurry. Additionally, the concentration of suspended solid particles within the slurry must be taken into account. For example, if the concentration of suspended particles in the slurry is low, additional mixing may be required to ensure adequate interaction between the suspended particles and the flocculating reagent. On the other hand, if the concentration of suspended solid particles is high, the influent stream of slurry may need to be diluted in order to obtain a lower concentration level for optimum flocculation of the slurry to occur.
Various methods have been used in the past to dilute the influent stream of slurry. For example, diluent, which may include clarified liquor obtained in the same or a previous sedimentation process, may be pumped to the feedwell and mixed with the influent stream of slurry to obtain a desired level of concentration. However, use of a pump requires additional piping, valving and monitoring equipment as well as additional energy. Such equipment, with the attendant operation and maintenance thereof, adds to the expense of the sedimentation process.
An alternative method of diluting the influent stream of slurry includes constructing the feedwell to include a dilution channel positioned below the level of the supernatant liquid in the settling tank such that an amount of clarified liquor spills into the dilution channel and mixes with the influent stream of slurry. However, with conventional spill-over methods, it is often difficult to accurately control the dilution ratio. This becomes particularly important as the concentration of solids in the influent stream changes during the process and adjustments to the dilution ratio must be made.
Another more recent method of diluting the influent stream of slurry includes the use of an eductor to effect mixing of the diluent with the influent stream of slurry as well as to control the dilution ratio. Such a method is set forth in U.S. Pat. No. 5,643,463, issued Jul. 1, 1997 (to Wood et al.), the disclosure of which is incorporated by reference herein in its entirety. The Wood et al. patent discloses a self-diluting feedwell which includes an eductor structure. The eductor structure, described therein, generally includes a walled channel including a launder portion for receiving the influent stream of slurry, at least one port for introduction of a diluent therethrough, a narrow through portion to bring about eduction of the diluent into the feed stream, and a discharge portion from which the diluted influent slurry is introduced into the basin.
One problem associated with the dilution method and apparatus described in the Wood et al. patent is that classification of the solid particles or sanding may occur when the influent stream of slurry is introduced at low flow rates. When sanding occurs, particles begin settling prior to the slurry being introduced into the settling tank, causing buildup on the floor of the walled channel within the feedwell. This in turn affects the flow characteristics within the feedwell, making it difficult to obtain the desired mixing, flow and distribution of the slurry into the tank. Additionally, such sanding may affect the concentration of the solids of the slurry entering the tank since, at high flow rates, the built-up sludge at the bottom of the walled channel may become disturbed, with the attendant result of solids being reintroduced into the stream of slurry. In effect, the method and apparatus of the Wood et al. patent are limited in their ability to effectively provide dilution in low flow situations.
Thus, it would be advantageous to provide a method and apparatus for diluting an influent stream of slurry which provides effective control of the diluent ratio without the need for extraneous and expensive mechanical equipment and which are not limited by the flow rate of the influent stream of slurry.